Here is an article from La Chambre des Notaires that explains very well the importance of a protection mandate :

« No one is immune to a serious accident, a degenerative disease that affects intellectual faculties or the effects of aging. If you found yourself in any of these situations, who would look after you and your property or assets? Many people believe that the spouse or children are automatically authorized to do so, but this is not the case.

Choosing the person who will look after you
The significance of a protection mandate in case of incapacity is, first, that it allows the mandator (i.e., the person who signs the mandate) to make an informed decision, effective immediately, about who will look after his or her well-being and administer his or her property or assets. The person entrusted with this task is known as the “mandatary.”

It is a great relief to know that a trustworthy person, such as your own spouse, brother, sister or close friend, will be legally empowered to make vital decisions for you.

Avoiding a family meeting and discussions
In the absence of a protection mandate in case of incapacity, family and friends must get together to choose a mandatary. In the case of a blended family, these discussions can be acrimonious. Just think what would happen if the spouse you have been separated from for 20 years and your children from your second marriage were gathered around the same table! Why entrust to others a decision you have every right to make yourself?

Avoiding financial worries and deadlines
If the breadwinner spouse has a stroke, for example, and is no longer able to manage his or her affairs, all his or her assets, including the bank account, are frozen. In this case things can quickly become complicated for the other spouse, who will be deprived of any financial resources. A protection mandate must take into account the family situation of each couple and be adapted accordingly.

Avoiding the involvement of the Public Curator
A protection mandate in case of incapacity allows you to avoid having your care turned over to someone you did not choose, such as the Public Curator, when you are no longer able to care for yourself.

Do not write your mandate by yourself! This is an important document that should be prepared with the greatest care. Otherwise, omissions could affect its validity and result in what you want to avoid at all costs: the institution of protective supervision.

If your protection mandate in case of incapacity is not notarized, ask your notary to review it. This will ensure that your document is complete and meets the conditions set by the law.»

Ref: http://www.cnq.org/fr/famille-couple/69-a-quoi-sert-le-mandat-de-protection-.html